Accepted answer

You're mixing up two different approaches, one binding data to the line and the other just passing the data to the line function. Either one could work (so long as you only have the one line), but if you want to get rid of your if/else construct you're still going to need to separate the statements handling entering/appending the element from the statements updating it.

Option 1 (don't bind data, just call the function):

var linegraph ='path');

if ( linegraph.empty() ) {
     //handle the entering data
     linegraph = channelGroup.append('path')
                      .attr("class", "line")
                      .style("stroke", channel.Color);

        .duration(animChart / 2)
        .attr('d', line(channel.DataRows));

Option 2 (use a data join):

var linegraph = channelGroup.selectAll("path")

                 .attr("class", "line");

        .duration(animChart / 2)
        .attr("d", line); //This is slightly more efficient than
                          //function (d) { return line(d); }
                          //and does the exact same thing.
                          //d3 sees that `line` is a function, and 
                          //therefore calls it with the data as the 
                          //first parameter.

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